My understanding of Christianity is that Jesus was the Son of God. As such Jesus fulfilled all of the prophecies of the Old Testament.
Why would there be a prophecy after the fulfillment of the greatest prophecy?
This provides people the ability to talk about the problematic history of prophecies while discouraging people from thinking that they are prophets.
Since the prophecies where fulfilled, anyone who claims to have a new gospel is misguided and is likely misleading their followers.
!WINE
I don't think these fake prophets claim to have a new gospel, they just tell people lies about their lives and future.. And unfortunately, people become gullible when they're desperate for something.